Styrene C.sub.6 H.sub.5 CH.dbd.CH.sub.2, is the common name for the simplest and by far the most important member of a series of unsaturated aromatic monomers. Sytrene is used extensively for the manufacture of plastics, including crystalline polystryene, rubber-modified impact polystyrene, acrylonitrilebutadiene-styrene terpolymer (ABS), styrene-acrylonitrile copolymer (SAN) and styrene-butadiene rubber (SBR).
Many different techniques have been investigated for the manufacture of styrene. The following methods have been used or seriously considered for commercial production: (1) dehydrogenation of ethylbenzene; (2) oxidation of ethylbenzene to ethylbenzene hydroperoxide, which reacts with propylene to give a-phenylethanol and propylene oxide, after which the alcohol is dehydrated to styrene; (3) oxidative conversion of ethylbenzene to a-phenylethanol via acetophenone and subsequent dehydration of the alcohol; (4) side-chain chlorination of ethylbenzene followed by dehydrochlorination; (5) side-chain chlorination of ethylbenzene, hydrolysis to the corresponding alcohols, followed by dehydration; and (6) pyrolysis of petroleum and recovery from various petroleum processes. The first two methods are the only commercially utilized routes to styrene: dehydrogenation of ethylbenzene accounts for over 90% of the total world production. Methods 4 and 5, involving chlorine, have generally suffered from the high cost of the raw materials and from the chlorinated contaminants in the monomer. Manufacture of styrene directly from petroleum streams (method 6) is difficult and costly.
The two commercially important routes to styrene are based on ethylbenzene produced by alkylation of benzene with ethylene.
Molecular sieves have been used in production of ethylbenzene which is subsequently dehydrogenated to produce the styrene. An ethylbenzene process was developed during the 1970s and was based on a synthetic zeolite catalyst, ZSM-5, developed by Mobil Oil Corporation. Although a number of zeolitic or molecular-sieve-type catalysts have been suggested for benzene alkylations with ethylene, most were characterized by very rapid build up of coke and, consequently, short-on-stream time. The Mobil catalyst represented a breakthrough in zeolite catalysis in that is combines high catalytic activity with relatively good resistance to coke formation. Total worldwide capacity based on the process was anticipated to be more than 3.times.10.sup.6 t/yr by 1985.
Molecular sieves include naturally occurring and synthetic zeolites. Certain of these zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by x-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are referred to as "molecular sieves" because interconecting channel systems created by pores of uniform pore size allow a zeolite to selectively absorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/ nO.Al.sub.2 O.sub.3.xSiO.sub.2.YH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated M. D. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p.5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula. One type of cation may be exchanged entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given aluminosilicate by suitable selection of the cation. The cavities and pores are occupied by molecules of water prior to dehydration and/or possibly by organic species from the synthesis mixture in the as-synthesized materials. The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979) and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), merely to name a few. The silicon/aluminum atomic ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with silicon/aluminum atomic ratios of from 1 to 1.5, while that ratio in zeolite Y is from 1.5 to 3. In some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. U.S. Pat. No. 3,941,871, reissued as U.S. Pat. No. RE. 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the x-ray diffraction pattern characteristic of ZSM-5 zeolites.